I'm interested in timeseries of numbers of owned dogs, cats, and possibly other pets (alligators, anyone?) for a wide cross-section of countries of the world. Primary focus is on the US.
You can get what you need for the US from this publication. But it is copyrighted material and expensive (~$300).
The all-new 2012 edition of the U.S. Pet Ownership and Demographics Sourcebook is the largest, most statistically accurate and complete survey of the pet owning public and pet population demographics. Drawn from a national survey of over 50,000 households, the survey results are presented alongside the results from similar surveys dating back to 1987, illustrating long-term trends.
Free (but Coarse):
The US Census publishes some very coarse information on pet ownership, but if you look where the data comes from it is the same source (American Veterinary Medical Association)
The Human Society has some coarse data as well:
The BLS has some coarse data as well:
Austin, TX - animal intake:
Colorado - animal intake:
Weatherford, TX - animal intake:
Adelaide, Australia - dog registration:
New Zealand - Dog Control Statistics